Edge blending allows you to create 1 continuous screen with 2 (or more) video projectors.
The principle is simple : the 2 video projectors overlaps a bit, so they blend together.
In this example, I'm using a Matrox DualHead, but it can work with another graphic card.

Here is the setup that takes a few minutes in Millumin :

Check that your DualHead is plugged to your Mac

Start Millumin and assign a media

Plug you DualHead to your video projectors.Then position your 2 video projectors so they overlap a bit.

Set up the output at the right resolution

Set up 2 screens (one per video projector)

Go to fullscreen (keyboard shortcut : ⌘+F)

You can now start to soft the edges.Adjust the smooth and the white point, to blend the best.

As you got an overlap zone, you are loosing some pixels on the right.This is normal.

Now you can adjust the gamma curve, the color or the keystone.

At last, readjust the smooth and white point to finish to soft the edges.